The present invention relates broadly to a voltage-controlled oscillator, and in particular to a phase-locked voltage-controlled oscillator apparatus for linear or complex frequency modulated waveforms.
It is well known in the prior art that an oscillator is an electronic circuit that converts energy from a direct current source into a periodically varying electrical output. If the output voltage is a sinewave function of time, the generator is called a sinusoidal, or harmonic oscillator. If the oscillator output waveform contains abrupt changes in voltage, such as occur in a pulse or square wave, the device is called a relaxation oscillator.
The fundamental laws governing sinusoidal oscillators are the same for all oscillator circuits. The amplifier provides an output voltage v.sub.o as a consequence of an external input signal voltage v.sub.s. The voltage vo is applied to a circuit which is called a feedback network and which has an output that is v.sub.f. If the feedback voltage v.sub.f were made identically equal to the input voltage v.sub.s, and if the external input were disconnected and the feedback voltage connected to the amplifier input terminals, the amplifier would continue to provide the same output voltage v.sub.o as before. This requires that the instantaneous values of v.sub.f be exactly equal at all times. Since no restriction was made on the waveform, it need not necessarily be sinusoidal.
If the entire circuit operates linearly and the amplifier or feedback network or both contain reactive elements, the only periodic wave that will preserve its form is the sinusoidal waveform, and such a circuit will be called a sinusoidal oscillator. For sinusoidal oscillators, the condition where v.sub.s equals v.sub.f requires that amplitude, phase, and frequency of v.sub.s and v.sub.f be identical. The phase shift introduced in a signal while being transmitted through a reactive network is invariably a function of the frequency, and there is usually only one frequency at which v.sub.f and v.sub.s are in phase. Therefore, a sinusoidal oscillator operates at the frequency for which the total phase shift of the amplifier and feedback network is precisely zero (or an integral multiple of 2). The frequency of a sinusoidal oscillator, provided the circuit oscillates at all, is therefore determined by the condition that the loop phase shift is zero.